Hydroxyethylmethylcellulose as a protective colloid in vinylacetate emulsion polymerization

ABSTRACT

AN IMPROVEMENT IN THE PROCESS FOR PREPARING VINYL ACETATE POLYMERS IN AQUEOUS EMULSION CMPRISING THE USE OF HYDROXYETHYLMETHYLECULLULOSE HAVING A THERMAL GEL POINT GREATER THAN 100*C., A HYDROXYETHYL MS OF FROM ABOUT 0.85-3.5, A METHOXY DS OF FROM ABOUT 0.4-1.6 AND A VISCOSITY AT 20*C. OF FROM ABOUT 2 TO 200 CPS., AS A PROTECTIVE COLLOID IN THE EMULSION POLYMERIZATION REACTION.

United States Patent Oflice 3,819,593 Patented June 25, 1974 U.S. Cl. 260-861 E Claims ABSTRACT OF THE DISCLOSURE An improvement the process for preparing vinyl acetate polymers in aqueous emulsion comprising the use of hydroxyethylmethylcellulose having a thermal gel point greater than 100 C., a hydroxyethyl MS of from about 085-35, a methoxyl DS of from about 0.4-1.6 and a viscosity at 20 C. of from about 2 to 200 cps., as a protective colloid in the emulsion polymerization reaction.

BACKGROUND OF THE INVENTION It is known that vinyl acetate monomer can be polymerized in aqueous emulsion in the presence of natural and synthetic water-soluble polymers which function as protective colloids in the polymerization process to provide products of desired particle size. Exemplary of such prior known protective colloids is hydroxyethylcellulose (HEC). This material, due to its hydrophilic nature, produces polymeric latexes of small particle size and good water resistant properties but with marginal freezethaw characteristics.

The water resistant properties of HEC are attributed to the ability of such material to become coordinated with the growing polymer chain. As the colloid becomes coordinated with the polymer chain, the relative surfactant action of such material increases thereby producing small latex particles. The freeze-thaw instability associated with the use of such colloids is due to the coalescing of the latex solids. It is an object of the present invention to provide, in the aqueous emulsion polymerization of vinyl acetate monomer, a protective colloid having reduced tendency to coordinate with the polymer chain and which will provide enhanced freeze-thaw stability without substantial reduction in latex water resistance. It is a further object to provide a means of obtaining a latex paint by the emulsion polymerization of vinyl acetate monomer, which paint is characterized by having improved enzyme stability along with improved flow and leveling properties without detriment to water resistance.

SUMMARY OF THE INVENTION The above and related objects are obtained by utilizing as a protective colloid in the aqueous emulsion polymerization of vinyl acetate monomer, of from about 0.05 to about 5 percent based on monomer weight, of a water-soluble hydroxyethylmethylcellulose having a thermal gel point greater than 100 C., a hydroxyethyl MS of from about 0.853.5, a methoxyl DS of from about 0.4-1.6 and a viscosity at 20 C. of from about 2 to 200 cps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The hydroxyethylmethylcellulose ethers utilized herein are ethers with a thermal gel point greater than about 100 C. and are inclusive of those materials as defined by US. Pat. No. 3,709,896 which have been shown to possess excellent enzyme stability in latex paint.

The terms molar substitution (MS) and degree of substitution (D8) are conventional terms describing substitution on the anhydroglucose units of the cellulose chain.

The ethers are prepared by reacting methyl chloride and ethylene oxide with alkali cellulose at about 40- 60 C. The alkali cellulose is prepared by treating cellulose with about 1.2-3.0 moles of 35-70% aqueous NaOH per mole cellulose at about room temperature. A dip tank as described in Savage US. Pat. 2,949,452, a spray mixer as described by Erickson US. Pat. 2,469,764, or a slurry reactor as described by Haskins, e.g., US. Pat. 2,131,733 are suitable. Contact with air is minimized to reduce viscosity loss.

Etherification is carried out in a pressure reactor in the absence of air at about 40-60 C. Because of the greater reactivity of the ethylene oxide, the reactor is generally charged with the major proportion of the methyl chloride at room temperature, heated to about 4050 C., and then the ethylene oxide added at a rate to maintain the desired temperature. However, incremental additions of methyl chloride and ethylene oxide can also be used. Normally, a total of about 1.0-6.0 moles of methyl chloride and about 3.4-7.0 moles of ethylene oxide are charged per mole of cellulose.

Careful control of the exothermic etherification is necessary to minimize irregular and uneven substitution. To maintain a temperature of about 40-60 C., external cooling means are employed. Excess methyl chloride can be used as a heat transfer and ebullient cooling agent. Other ebullient diluents such as dimethyl ether or a water-soluble organic liquid such as isopropanol or tertiary butanol can also be used to provide more eifective heat transfer and moderate the rate of etherification.

When the exothermic hydroxyethylation is completed, the reaction can be finished at 55-80 C. Without harm. The overall reaction time may be 4-12 hours. Then the reactor is vented, excess caustic is neutralized, and the product purified by appropriate means.

Since the hydroxyethylmethylcellulose ethers are watersoluble but nonthermal gelling below C., the byproducts are removed by washing with appropriate organic solvents or limited amounts of aqueous salt solutions. Particularly useful in minimizing the loss of the water-soluble product is the glyoxal insolubilization process described in US. Pats. 3,347,847 and 3,527,751. After purification, the product can be dried, granulated, ground, surfacetreated, or otherwise prepared for storage and end use.

As described herein, the prescribed hydroxyethylmethylcellulose ethers are particularly effective as protective col; loids for the polymerization of vinyl acetate monomer or mixtures of predominate amounts of monomeric vinyl acetate with one or more other monoethylenically unsaturated monomers. Exemplary of particularly preferred other such monomeric materials which may be copolymerized with predominant amounts of vinyl acetate according to the process of the present invention are the acrylates and methacrylates including the methyl, ethyl, propyl, butyl and 2-ethylhexyl acrylates and methacrylates. Other monomers which may be advantageously polymerized or interpolymerized according to the process of this invention are vinyl propionate, the vinyl and vinylidene halides such as vinyl chloride and vinylidene chloride, acrylonitrile and methacrylonitrile.

Obtainment of desired polymer particle size may be realized utilizing the prescribed cellulose ethers in a concentration ranging generally from about 0.05 to 5 percent based on the weight of monomer.

The choice of viscosity will depend upon the system used. For preparation of polyvinyl acetate and copolymers of vinyl acetate with an acrylate, e.g., 2-ethylhexylacrylate,

EXAMPLE 1 (A) Preparation of Hydroxyethylmethylcellulose The following materials were reacted as described below, in a 55 cubic foot roller blender reactor:

Conc., percent Weight (lbs.)

Material Alkali cellulose NaOH 50 Ethylene oxide Citric acid Glyoxal Alkali cellulose, prepared from the reaction of cellulose with sodium hydroxide, was degraded in the reactor for 2 /2 hours at 70 C. The reactor was then cooled to C., evacuated and methyl chloride added. The reactor was then heated to C. over a period of 1 hour after which time ethylene oxide addition began. During the hydroxyethylation reaction the temperature was maintained at 45 C. with an ethylene oxide addition rate of 2.5 pounds per minute. The reaction was continued until the residual alkali was present at a 2% concentration. The reactor was then vented. After venting, the reactor was cooled to 35 C. and evacuated. Citric acid was added to achieve a pH of 5. The glyoxal was then added and the reactor heated to 80 C. until residual moisture content was about 20%. The reactor was then vented and emptied. The crude product was washed with water (3 :1 ratio of water to solids). The purifiied product was dried to 5% moisture and ground.

(B) Preparation of emulsion polymerization of vinyl acetate The following is a typical formulation for the preparation of an emulsion polymerizate of vinyl acetate monomer:

Percent by emulsion Deionized water Protective colloid Sodium bicarbonate (bufler) Ammonium persulfate (initiator) Non-ionic Surfactant (Igepal 00-897, 70%

Monomer l 2.5% based on monomer weight.

weight of 0 4 control, condenser and an addition tube leading from the monomer reservoir through a syringe pump. The syringe pump was calibrated to deliver the monomer continuously over a 2-hour period. Agitation of the emulsion was fast enough to avoid any surface pooling of monomer. Proper agitation also determines the original distribution of monomer between the droplets and micelles and maintains adequate heat transfer during the initial exotherm. As the monomer addition took place, the immersion temperature increased to 72-75 C. After the 2-hour addition time, the batch temperature was raised to C. over a 30- minute period and maintained for 30 minutes to reduce residual monomer to less than 1%. The emulsion was then cooled to room temperature and poured through a 60-mesh screen into a suitable container. The insolubles retained on the screen were then recorded as coagulum.

(C) Latex Testing Procedures and Results In each of a series of experiments, an aqueous emulsion polymerizate containing vinyl acetate monomer was tested as follows:

Freeze-Thaw Stability.--Two hundred grams of the latex were placed in a resin lined half-pint paint can and put in the freezer for 24 hours at 0 C. The sample was removed and stored at room temperature for 8 hours. The cycle was repeated until total latex coagulation occurred or five cycles were completed.

Mechanical Stability.Two hundred grams of latex were put on a Hamilton Beach milk shake mixer. The sample was run until coagulation occurred or for a total of 30 minutes. The material was then passed through a US. 40 mesh screen and the residual was collected and rated on a scale from 1-10 (1-trace amounts, 10'excessive insolubles 1-2 gm.).

Heat Stability.0ne hundred grams of latex were put in a 4-ounce bottle, capped tightly and stored at F. for one week. The samples were then evaluated for gelation and phase separation.

Coagulum.The total latex system was poured through a 60 mesh screen and residual collected and reported as coagulum by the rating system applied as described above.

Percent Conversion.A 10 gram sample of the finished latex was weighed into a tared aluminum pan. The sample was then dried in an oven at 200 F. to a constant weight.

Actual solids A percentage correction for percent conversion was taken into account for the coagulum already removed.

Viscosity.Brookfleld Model LVF was used with a number 3 spindle at 30 r.p.m. at room temperature.

Initial pH.The pH of this finished latex was determined the day following preparation using a Sargent single electrode pH meter. For optimum emulsion stability, the pH should be between 4 and 5.

Film Properties.(a) ClarityVisual comparison as to haziness, fibers and insolubles in a film cast onto sheets of ethylcellulose.

(b) AdhesionComparison of the adhesive properties of the film on the ethylcellulose sheeting was done by slicing the film with a razor blade. The amount of pullaway from the sheeting was rated either excellent, good, fair or poor.

(c) ScrubabilityA 10 mil (wet) film was cast on a polyvinyl chloride test sheet (Lenetie black plastic scrub panel). The film was allowed to dry for 48 hours at ambient temperature. Since the percent solids varied only slightly, no dry film thickness measurements were taken. The dried films were placed in a Gardner Heavy Duty '5 WeanTestengTe'n grams of abrasive scrub media were added 10's nylon brush which had been presoaked for minutesin a 1% solution of Triton X-l00. Five mils. of ivjaft rflwejre acea in the brush path and the scrubbing 6 EXAMPLE 2 The Preparation of Paint Formulations from Vinyl Acetate Polymer Latexes Prepared in the Presence of 3. Protective Colloid cycles started. After 400 cycles another gms. of scrub 5 media and 5 mls. of water were added. The first com- Th f n t t l I l t t plete film break was taken as the end point. 9T? i i a yplca Po yvmy ace a 6 Particle Size Determinations -Latex samples were c Pam 0mm a diluted with distilled water and .treated with one drop of Lgs [100 1.0% Uranyl Acetate per 2 ml. of dilute latex. One drop 10 Grams gal. sample was dried on a carbon coated substrate for exami- 1,200 300 nation and photomicrography. Results reported were num- 53 g? n w age? 80) 2; 5 her average particles size (microns, t) and distribution Preijwiti itfi tti fl l fi 5 ,95

- Non-ionic surfactant I e al 0-630 6 4 range of particle sues Ethylene glyeoluunff p 100 The following Table I (Section A) sets forth properties %Q(f)OH3(T II1T(NODC) 1 25(21 ipure90 1 obtained by utilization of a series of hydroxyethylmethyl- Calcium carbonate (Snow Flake White) 400 100 cellulose materials in the emulsion polymeriza- Clay filler 500 125 t ion of vinyl acetate monomer to produce a polyvinyl ace- Above is known as the pigment grind eigo n r tex S pfl I Sets forth 20 coales ent T.x.n.,1 64 16 parative data using conventional hydroxyethylcellulose PVAc 1am 1.000 250 Ipaterials protective collokl 3% Aqueous solution of protective colloid 380 95 TABLE I H I A (the invention) B (for comparison) Sample Nos. 1 2 3 4 5 6 7 Colloid viscosity I v cps 40 cps. 48 cps. 19 ens 19 cm 14 cps. 13 cps m m {D8, .5 DS,.58 DS, .86

MS, 1.76 MS, 1.76 MS, 1 as Ms, 2 5 Ms, 2 5 Ms, 1 a MS, 1.8 Freeze-thaw 5 cy l 4 cycles..-.--. 3 cycles 2 cycles 2 cycles 1 cycle 2 cycles Mechanical stability Insol.,5 Insol. 1 Heat stabi1ity-- Good. Good Coagulum-.- Insol., 1 Insol., 1 Percent conversion 99 5 99.3. Viscosity (cps.) 452 1,360 pH- 4 4 4.6- Film clarity Poor Goo Scrub. data (cycles) 5s 39 Particle size (microns) lranael n 8 0,5

l Dialyzed.

The foregoing data illustrate that significantly improved latex freeze-thaw stability is achieved using the HEMC protective colloids as compared to utilization of the conventional HEC protective colloids.

The following Table 11 sets forth properties obtained by utilization of a series of hydroxyethylmethylcellulose materials as protective colloids in the emulsion polymerization of a monomer mixture composed of 85 percent The ingredients which make up the pigment grind were mixed on a Cowles mixer at 4400 r.p.m. Texanol and a 3 percent solution of thickener were then added to the grind and mixed with low speed agitation. The resulting mixture was then added to the various PVAc latexes, mixed for 30 minutes and left for 24 hours for viscosity build. Viscosity and heavy duty scrubability properties were measured as described supra. Flow and leveling vinyl acetate and 15 percent 2-ethylhexyl acrylate. properties of the paint were determined by comparison TABLE II Sample number 8 9 10 11 12 13 Co o d viscosity p g g f gg eggp 6 l 2 Subsflmfl --"-----{MsI 1116.1-.-11'. Ms, 1:76::::::: Ms; iai. I. s1 I Freeze-thaw. 2 cycles. 4 cycles. 2 cycles. 4 cycles.

Mechanical stability. Insol., 5 Insol., i Insol., 6-7 Insol., 10

Heat stability- Good- Goodood. ood- Coagulum Insol., 7 Trace amounts Insol., 10+++ I $01., 5 Insol 3.

Percent conversion- 99 1 99.6 99.5 99

Viscosity (cps.) 7 650 420 1,520.

Film clarity Goodwinliant Good- Excellent Excellent Excellent.

Arlhaelnn Excellent Excellent Fxr-allcnt Excellent Fxvellanf Excellent,

Scrub data 215 274 86 111 132 159.

Particle size (microns) Imag .7 (3-1.2 .6 (2-1.1) .4 (.l-1.1) .3 (.1-.8) 7. (.4-L2) .5 (2-1.1).

Sample number 14 15 16 17 18 Colloid viscosity (cps.) 2.6-

Substitution Freeze-thaw- Mechanical stability- Heat stability Percent conversi n Viscosity (cps.)

Film clarity Fxcellent Excellent Good- Good Good.

Adhesio Excellent Farr-client Excellent "Excellent Excellent,-

Scrub. data 101 1 09 148 181. Particle size (microns) [range] .3 (.15-.8) .6 (3-1.1) .7 (3-1.2) .5 (.3-.9) .5 (3-1.0).

7 of paint brush outs with standards. To prepare brush outs, 20 gms. of test paint were brushed onto a two foot square area and allowed to dry. The rating is 1 to with 1 being the poorest and 10 the best.

The following Table I I I sets forth paint properties achieved using a series of protective colloids:

2. The process of Claim 1- wherein said monomer is'fa 260-855 N, 85.7,.875 R, 87.7, 88.7 F, 89.1,;911,

I f r PM :3: -vi mixture of predominant amounts of vinyl acetate and correspondingly lesser amounts ofa monoethylenically unsaturated comonomer...

TABLE III Protective colloid Visc. of Scrub. Flow substitution paints data, and Vise. (Kreb days leveling j Sample number Type DS MS (ops) units) (cycles) .mting For comparison:

19 HEC 2.5 19 79 1,150 5 HEC 1.8 13-14 83 1,111 5 The invention:

21..-. .58 1.76 78 1,090 7 22.-.. HEMC (dialyzed)- .58 1.76 40 68 1,090 7 23-. .42 1.29 8.7 1,020 6 24 H .44 2.19 40 1,620 7 1 1=worst; 10=best.

The above data illustrate that polyvinyl acetate based paints, wherein hydroxyethylmethylcellulose (HEMC) (as 20 prescribed by this invention) was used as the protective colloid in the polymerization of the vinyl acetate monomer, have equivalent or improved water resistance and improved fiow and leveling properties as compared to polyvinyl acetate based paints containing the same amounts of conventionally used hydroxyethylcellulose (HEC) ma- 3. The process of Claim 2 wherein said. monoethylenically unsaturated .coinonomer is selected-from the group consisting of methyl,gethyl,' propyl, butyl' and. 2 ethylhexyl acrylates and methacrylates. ,1:

4. The process of Claim 3 wherein said monomer is a mixture of vinyl acetate and 2-ethylhexyl acrylate.

5. The process of Claim 4 wherein said monomer is a mixture of about'85'percentby'wight'vinyl'acetate and about 15 percent by weight Z-ethylhexyl acrylate.

References Cited f UNITED, STATES PATENTS 2,587,562 2/1952' Wilson 26o-;-'89.1 3,205,204 9/1965 l-Iec-kmaier etal. 26o 9 2.sw

35 HARRY WONG 111., Primary Examiner us. 01. x11, 

